Devices for regulating the angular speed of a wheel, also called rotors, via a magnetic coupling, also called a magnetic connection, between a resonator and a magnetic wheel, have been known for many years in the field of horology. Several patents relating to this field have been granted to Horstmann Clifford Magnetics Ltd for the inventions of C. F. Clifford. In particular, U.S. Pat. No. 2,946,183 may be cited. The regulating devices described in these documents have various drawbacks, in particular a problem of anisochronism (defined as non-isochronism, i.e. a lack of isochronism), namely a significant variation in the angular speed of the rotor as a function of the drive torque applied to the rotor. The reasons for this anisochronism have been incorporated in the developments leading to the present invention. These reasons will become clear hereafter upon reading the description of the invention.
There are also known from Japanese Patent Application No JP 5240366 (Application No JP19750116941) and Japanese Utility Models JPS 5245468U (Application No JP19750132614U) and JPS 5263453U (Application No JP19750149018U) magnetic escapements with direct magnetic coupling between a resonator and a wheel formed by a disc. In the first two documents, rectangular apertures in a non-magnetic disc are filled with a highly magnetically permeable powder, or a magnetized material. There are thus obtained two annular, coaxial and adjacent paths, which each include rectangular magnetic areas regularly arranged with a given angular period, the areas of the first path being offset or phase shifted by a half-period relative to the areas of the second path. There are thus obtained magnetic areas, alternately distributed on either side of a circle corresponding to the position of rest (zero position) of the magnetic coupling element or member of the resonator. This coupling member or element is formed by an open loop, which, according to the case, is made of magnetized or highly magnetically permeable material, between whose ends the disc is driven in rotation. The third document describes an alternative wherein the magnetic areas of the disc are formed by individual plates of highly magnetically permeable material, with the magnetic resonator coupling element then being magnetized. The magnetic escapements described in these Japanese documents do not enable isochronism to be significantly improved, in particular for reasons which are explained below with the aid of FIGS. 1 to 4.
FIG. 1 is a schematic view of an oscillator forming a magnetic escapement 2 of the type described in the aforementioned Japanese documents, but already optimised in that the magnetic teeth 14 and 16 of the wheel 4 define annular sectors which each extend over a half-period of oscillation and in that a coupling element with a round or square end is selected for the resonator, to better allow comparison with an embodiment of the present invention shown in FIG. 5 and to demonstrate objectively the benefits of the present invention. Wheel 4 includes a first series of teeth 14, respectively separated by a first series of holes 15, which define together a first annular path. This wheel further includes a second series of teeth 16, respectively separated by a second series of holes 17, which define together a second annular path. Teeth 14 and 16 are formed by a highly magnetically permeable material, in particular a ferromagnetic material. The two series of teeth are respectively connected by an outer ring 18 and an inner ring 19 formed of the same magnetic material. The two annular paths are adjacent and delimited by a circle 20, which corresponds to the rest position of magnet 12, located at the centre thereof, of resonator 6 for every angular position of wheel 4, i.e. to the position in which the resonator has minimum elastic deformation energy.
The resonator is symbolically represented by a spring 8, corresponding to its elastic deformation capacity defined by an elastic constant, and by inertia 10 defined by its mass and structure. The resonator is capable of oscillating at a natural frequency in at least one resonant mode wherein magnet 12 oscillates radially. It will be understood that this schematic representation of resonator 6 means, within the scope of the invention, that it is not limited to a few specific variants. The essential is that the resonator includes at least one magnetic coupling element 12 for magnetically coupling the resonator to the magnetic structure of wheel 4, which, in the example shown in FIG. 1, is driven in rotation by a drive torque in the anticlockwise direction at angular speed ω. Magnet 12 is thus located above wheel 4 and is capable of oscillating radially about the zero position located on circle 20. Since magnetic teeth 14 and 16 form areas of magnetic interaction located alternately on either side of central circle 20, they define a wavy magnetic path with a determined angular period Pθ, which corresponds to the angular period of each of the first and second angular paths. When the resonator is magnetically coupled to the wheel, so that magnet 12 oscillates along the wavy magnetic path defined by the wheel, the angular speed ω of the wheel is substantially defined by the resonator oscillation frequency.
FIG. 2 is a schematic view, on one portion of wheel 4, of the magnetic potential energy (also called magnetic interaction potential energy) of oscillator 2 which varies angularly and radially according to the magnetic structure of the wheel. The level curves 22 correspond to various magnetic potential energy levels. They define equipotential curves. The magnetic potential energy of the oscillator at a given point corresponds to the state of the oscillator when the magnetic resonator coupling element is in a given position (its centre being located at this given point). It is defined to within one constant. In general, magnetic potential energy is defined with respect to a reference energy which corresponds to the minimum potential energy of the device concerned, in this case the oscillator. In the absence of dissipative force, this potential energy corresponds to the work necessary to bring the magnet from a minimum energy position to a given position. In the case of the oscillator concerned, the work is provided by the drive torque applied to wheel 4. The potential energy accumulated in the oscillator can be transferred to the resonator when the magnet returns to a lower energy position, in particular a minimum energy position, by a radial movement relative to the axis of rotation of the wheel (i.e. according to the degree of freedom of the useful resonant mode). In the absence of dissipative force, this potential energy is converted into kinetic energy and elastic energy in the resonator by the work of the magnetic force between the resonator coupling element and the magnetic structure. This is how the drive torque supplied to the wheel is used to maintain the resonator oscillation which in return brakes the wheel by regulating its angular speed.
The outer annular path defines alternating areas of minimum energy 24 and areas of maximum energy 25 while the inner annular area defines, with a phase shift of an angular half-period Pθ/2 with respect to the first path (i.e. a phase shift of 180°), alternating areas of minimum energy 28 and areas of maximum energy 29. FIG. 3 shows two outlines 32 and 34 giving the position of the centre of magnet 12 when oscillator 2 is operating and when wheel 4 is thus driven in rotation with angular speed regulation. These outlines are thus a representation of the oscillation of the magnet with two different amplitudes within a reference frame linked to the wheel. An examination of the magnetic potential energy level curves 22 and the oscillations 32 and 34 reveals that the oscillator accumulates magnetic potential energy with each vibration in accumulation areas 26 and 30. The force exerted on the resonator magnet is given by the magnetic potential energy gradient, this gradient being perpendicular to level curves 22. The angular component (degree of freedom of the wheel) works by reaction on the wheel while the radial component (degree of freedom of the resonator) works on the resonator coupling member. In the accumulation areas, the angular force corresponds to a braking force of the wheel since the angular reaction force opposes the direction of rotation of the wheel. When the magnetic force is essentially angular in the accumulation areas, the accumulation of magnetic potential energy accumulation in the oscillator is said to be “pure”.
In FIGS. 2 and 3, the pure accumulation areas define substantially annular areas Z1ac* and Z2ac*. The accumulated energy is then transferred to the resonator in a central impulse area ZCimp*. In central area ZCimp* and, more precisely, in the impulse areas where the oscillations of the magnet pass, the magnetic potential energy gradient has a radial component which gradually increases with rotation of the wheel, whereas the angular component decreases to eventually become zero. This gradient corresponds to a thrust force for the magnet and thus to an impulse. When the amplitude is relatively high (oscillation 32), it is noted that the thrust force is applied over the entire width of the central area between points PE1 and PS1. For a lower amplitude (oscillation 34), the passage through central area ZCimp* extends over a greater angular distance between points PE2 et PS2 and, in the first half of the crossing of the central area (approximately as far as central circle 20), the oscillation is substantially free, a lower energy impulse being given only in the second half of the crossing.
Generally, an “accumulation area” means an area in which the magnetic potential energy in the oscillator increases for the various oscillation amplitudes of the useful drive torque range; and an “impulse area” means an area in which this magnetic potential energy decreases for the various oscillation amplitudes of the useful drive torque range and where a magnetic thrust force is exerted on the resonator coupling member along a degree of freedom. “Thrust force” means a force in the direction of motion of the oscillating coupling member. Thus, although this thrust force may already exist in an accumulation area, this description will refer to impulse areas as being outside the accumulation areas.
To understand the level curves 22 shown in FIGS. 2 and 3, it is necessary to consider an important aspect of the embodiment of oscillator 2 for it to be functional. In particular, in the field of horology, the drive torque supplied by a barrel varies significantly as a function of the mainspring tension level. To ensure that the timepiece movement works over a sufficiently large period, the movement is generally required to be able to be driven by a torque varying between a maximum torque and approximately half the maximum torque. Moreover, it is of course also necessary to ensure proper operation at maximum torque. In practice, to ensure such operation and prevent, in particular, the oscillator becoming uncoupled at a relatively high oscillation amplitude, braking areas 26 and 30 are required to extend over a certain angular distance and braking must thus be gradual. This situation is obtained partly, and in a non-optimum manner, with background art oscillators by an averaging effect essentially resulting from the angular extent of the magnetic coupling member or element of the resonator in projection in the general plane of the wheel, and from the relatively large air gap between this member and the magnetic structure of the annular paths of the wheel (more generally of the rotor or rotating wheel set).
The averaging is obtained by integration over the entire coupled magnetic field, which extends over an area of the magnetic structure, whose size increases with the size of the end surface of the magnet parallel to said general plane and with the size of the air gap. Thus, the vertical flank of a magnetic tooth adjacent to an opening in the magnetic structure concerned, in the magnetic potential energy space, gives level curves 22 which extend over an angular distance which increases with the averaging effect. The case analysed here used a magnet having a circular or square section parallel to the general plane of the wheel. The dimension selected for this section and the selected air gap already provide a more favourable arrangement than those of the aforecited background art devices for operation of the oscillator, since brake pads 26 and 30 are ensured to be sufficiently extensive while already slightly limiting the radial distance of the central impulse area.
When the behaviour of the oscillator considered above is analysed according to the drive torque applied to the wheel, there are observed at least two drawbacks of such a regulating device. First of all, the range of values for the drive torque is relatively reduced and there is significant anisochronism. This is shown in the graph of FIG. 4, which shows the relative angular speed error (ω−ω0)/ω0 of wheel 4, (ω0 being the nominal angular speed) relative to the relative torque Mrot/Mmax applied to the wheel (for a resonator quality factor of around 200). Angular frequency ω0 is mathematically linked to the natural frequency Fres of the useful resonator oscillation by the formula ω0=2πFres/NP, NP being the number of angular periods of the first and second annular paths. The various points 36 define a curve 38 corresponding to a high anisochronism for a timepiece application. Indeed, a relative error of 5·10−4 corresponds to a very significant daily rate error, namely around forty seconds (40 s). Next, instability is observed in the oscillator behaviour when the relative torque is close to 80% (0.8), as evidenced by point 40. Thus, to obtain accuracy of less than ten seconds per day for the timepiece movement, the relative torque must remain within a narrow range of between 0.6 (60%) and 0.8 (80%). In practice, the timepiece movement must be devised so that the maximum acceptable torque corresponds to the maximum torque applied to wheel 4, so that torque will have to remain above 80% in this practical case. As soon as this lower limit is approached, the anisochronism increases rapidly and becomes enormous once the lower limit is passed. This explains one significant reason for the lack of success of such magnetic escapements although they have been known for dozens of years.